Technically I think your distributor can order them from Starboard. If you're used to doing repairs, it should be a rather simple operation. The boxes come pre fabricated in a hard foam block. So you need to route out a rectangular hole and then cut the foam until you reach the top laminate. WHen the boxes are installed from scratch you put them right through the board, but when retrofitting I would keep the top laminate intact.
Making the box fit nicely with the top laminate shape will require some measuring and careful sanding, but probably you can do it quite roughly and fill the cavity with expanding pu foam when mounting the box. Just make sure the foam doesn't push the box out of the board. when it expands. You don't need anything else than epoxy with some micro balloons in it for the mounting (ie no need for fiber). The upper surface of the box consists of some fibre layers and it's important that it comes out flush with the bottom (or a tiny weenie bit below). Then just smooth out with some filler, sand and put a layer of thin fibreglass over it all. I reckon it's a few hours work, but cut up in many short sessions.
But first, ask your dealer to as your distributor to order. It might take some time. While you're at it you could do the front boxes too and then enjoy the full tunability of the IQ system.
My local custom builder will start making his own slot boxes soon, so if your dealer hesitates, post here again and I can see what I can do about a set of custom boxes.